Signaling-circuit.



G. A. CAMPBELL.

' SIGNALING CIRCUIT. APPLICATION HLED SEPT-9. 1916.

1,254,473. Patented Jan. 22,1918.

9 SHEETSSHEET 1- WUNESBSES} IIVVENTOR Jwl: t 2/ Geo/ye fl. Campbell W In I A TTORNEY G. A. CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED SEPTA? 19m.

1,254,473. Patented Jan. 22,1918.

9 SHEETS-SHEET 2- W/TNESfiES: 7 INVENTOR G60Zy6 fl. Campbell A TTORNE YG. A. CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED sans. 1916.

1,254,473. Patented Jan. 22,1918.

9 SHEETS-SHEET 3.

By k

ATTORNEY G. A. CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED SEPT.9.1916.

1,254,473. Patented Jan. 22,1918.

9 SHEETSSHEET 4.

INVENTOR W/TIVESSELS H I 060796 fl. Campbell &" l ATTORNEY G. A.CAMPBELL.

SIGNALING CIRCUIT.

APPLICATlON HLED SEPT.9. 1916.

1 ,254,473 Y Patented Jan. 22, 1918.

9 SHE'ETSSHEET 5.

WITNESSES: INVENTOR Georye J4. Cam bell BY 1 M A TTOR/VEY G. A.CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED sEPT.9.191s.

1,254,473. Patent-ed Jan. 22,1918.

9 SHEETS-SHEET 6- W/TNESgES; INVENTOH r1 .5 3 5 p Gearge fl. Campbell 14TTORN E Y G. A. CAMPBELL.

SIGNALING CIRCUIT.

APPUCATION FILED SEPT.9. 1916.

1,254,473. Patented Jan. 22,1918.

9 SHEETSSHEET 7- W/T/VESSE 1 IN VENTOR ATTORNEY I Ai 1775M Geof 9 J4.Campbell E6111; R T

G. A. CAMPBELL.

SIGNALING CIRCUIT.

APPLICATION FILED SEPTA]. 191a.

Patented J an. 22, 1918.

9 SHEETS-SHEET 8.

L N I Nail);

Fig. 24 Na X ATTORNEY UNITED STAT s ion.

GEORGE A. CAMPBELL, or MONTCLAIB, NEW JERSEY, ASSIGNOR 'ro-sivisnroanTELE- PHONE AND TELEGRAPH COMPANY, A oonronarron on NEW some s ISIGNALING-CIRCUIT.

To all whom it may concern: Be it known that I, Gnoaen A. CAMPBELL,residing at Montclair, in the county of Essex and State of New Jersey,have invented certain Improvements in Signaling-Circuits, of

which the following is a specification.

This invention relates to circuit arrangements for signaling systemswherein signals may be either transmitted from or received at the samestation. In its more specific aspects this invention is embodied in asubscriberstelephone station,hereinafter termed, in accordance withcommon usage, a substation, and more particularly in the combination ofa substation and a telephone line. Its object is to provide a signalingcircult arrangement which in cooperative combination with a similar andequal communicating arrangement or station shall deliver the max- 'imumamount of energy to the receiving apparatus of said communicatingstation or arrangement. A further object is to provide an arrangementsuch that the receiving apparatus is-protectejd from interference by thetransmission energy originating at the same station. In other words itsobject is to provide signaling means characterized by the maximumpossible ratio of received and transmitted energy and furthercharacterizedby the absence of. side tone.

. The object of the invention is attained, in its specific aspect, byproviding a substation consisting of transmitter, receiver, auxiliaryresistance, and a transformer hav-v ing a plurality'of windings which,in combination with a telephone line, shall satisfy the followingfundamental requirement: Given two identical substations designed forinvariable two-way communication, and con- 40 nected by a line of givenimpedance and length, the amount'of energy absorbed by the receiver atthe receiving station shall be the maximum part of the total telephonicenergy developed by the transmitter at the transmitting stationconsistent with invariable two-way communication, and, as hereinafterexplained, consistent with a desirable amount of discrimination againstdisturbing line noise. This fundamental requirement may be stated interms of the following subordinate requirements which are necessary forits satisfaction: (1) The transmitter and receiver shall be conjugate,that is there shall be negligible side tone in the receiver inconsequence of the actuation of the Specification of Letters Patent.

Patented Jan. 22, 1918.

Application filed September 9, 1916. Serial No. 119,284.

transmitter by sound waves; (2) the line and auxiliary resistance shallbe conjugate in order that none of the energy absorbed by the substationfrom the line shall be wasted in said auxiliary resistance; (3) for agiven line having a definite impedance the telephonic energydelivered'by the transmitter shall be a maximum; (4) the amount ofenergy delivered by the line to the substation shall be a maximum, inother words the impedance of the substation as seen from the line shallbe equal'to the impedance of the line; (5) at a small sacrifice ofeificiency it shall be possible to discriminate effectively againstdisturbing line noise as distinguished from the telephonic signals fromthe communicating station.

A substation satisfying the above-mentioned requirements is ideal inthat its overall efliciency from transmitter of onesubstation toreceiver of the communicating substation is a theoretical maximum whichcannot be exceeded by any invariable substations whether satisfying therequirement of transmitter and receiver conjugacy or not. It is furtherideal in the sense that a minimum number of elements is employed sinceat least one auxiliary element is necessary to secure freedom from sidetone.

It might be inferred that the addition of an auxiliary resistanceelement, necessary as it is to secure freedom from side tone, would atthe same time necessarily reduce the efficiency of the substation sinceenergy is unavoidably wasted in said auxiliary resistance. That this isnot the ease and that the efficiency of the substation of my inventionis a theoretical maximum which cannot be exceeded by any two-waysubstation whether with or without side tone, the followingconsiderations will show. The simplest form of substation for invariabletwoway communication is that in which the receiver and transmitter areconnected in series with each other across the line. In such anarrangement the over-all efiiciency is a maximum when the resistance ofthe receiver is equal to that of the transmitter. When this condition issatisfied obviously fifty per cent. of the energy delivered by the lineto the substation is wasted in the transmitter and fifty per cent. ofthe energy delivered by the transmitter is wasted in the receiver.Further such an arrangement labors under the disadvantage of'full sidetone. In the 110 resistance and said line are conjugate; hence theefficiency is as great as that of the simple series substation. Whentransmitting no en-,'

ergy is wasted in the receiver but fifty per cent. of the energydelivered by the transmitter is wasted in the auxiliary resistance. Thetransmitting efiiciency is therefore also a theoretical maximum andneither transmitting nor receiving efliciency is reduced by the additionof the auxiliary resistance which is necessary to secure freedom fromside tone. The foregoing consideration will serve to explain thedesirability of having the line and auxiliary resistance conjugate aswell as the transmitter and receiver.

7 In my present invention I provide a substation comprising transmitter,receiver, auxiliary resistance and transformer and so proportion saidcomponent elements and so relate them to a telephone line and to eachother that, in combination with said telephone line, said substationsatisfies all of the foregoing requirements.

I have discovered that the above-mentioned requirements may be satisfiedby a large number of arrangements employing the minimum number ofelements and all equally eflicient and without side tone. Whiletheoretically all these arrangements are equally good, practicalconsiderations make certain arrangements referable.

My invention Wlll now e fully understood by rreference to theaccompanying drawing in which:

Figure 1 is a diagram representing one form of my invention, while Figs.4 and 7 are modifications in which the transformer connections areinterchanged.

Fig. 10 is a diagram of a second form of my invention, of which Figs. 13and 16 represent modifications produced by varying the transformerconnections.

Fig. 19 is a diagram of a third general form of my invention, Figs. 22and 25 being diagrams of modifications produced by changing thetransformer connections.

Figs. 2, 5, s, 11, 14, 17, 20, 23 and 2e are diagrams illustrating thecurrent flow in Figs. 1, 4, 7, 10, 13, 16, 19, 22 and 25 respecrom thesubstation.

Figs. 3, 6, 9, 12, 15, 18, 21, 24 and 27 are diagrams showing thecurrent flow in Figs. 1, 4, 7, 10, 13, 16, 19, 22 and 25 respectively,where signals are being received by the substation.

Fi s. 1(a), 4(a), 7(a), 10(a), 13(a), 16(a, 19(a), 22(a), and 25(a) areschematic diagrams of the substations of Figs. 1, 4, 7, 10, 13, 16, 19,22 and 25r'espectively,

the diagrams being simplified and so am tively, during the transmissionof signals 55 f teasers ranged as to indicate more clearly theequivalence of the several modifications.

In order to illustrate the scope of my invention and elucidate theprinciples on which all specific embodiments-rest, a general theoreticaldiscussion will now be ven which-applies to all substations satisfyingthe requirements heretofore stated in this specification. In thisdiscussion and the equations and formulae included in this specificationthe subscripts 1, 2, 3 and 4 will refer to transmitter, receiver,auxiliary resistance and line respectively. Thus 1,, I I I, will denotethe currents flowing in transmitter, receiver, auxiliary resistance andline respectively, while R, will denote the resistance of thetransmitter, R, the resistance of the receiver, etc. i 1 Consider asubstation consisting of trans ,mitter, receiver, auxiliary resistanceand appropriate transformer windings, connected to a line of givenimpedance. In practice the line connects two similar and equalsubstations between which communication is established. It is a wellknown principle that if a terminal impedance is connected to a source ofelectromotive force through a line of impedance ance component, theimpedance of the ter-.

minal arrangement as seen from the line should be equal to theresistance component of the impedance of the then, that the substationshall have maxiline. The condition,

mum energy absorption from the line is that its impedance. as seen fromthe line, shall be equal to the line impedance. The significance of theforegoing statement may be explained by reference to Fig. 1 as follows:Let the substation be disconnected from. the line and let the impedanceof the substation be measured across terminals 8 and 5-. Then theimpedance so measured shall be equal to the impedance of the line. Withthe line terminated at each end by a substation satisfying thiscondition, the line may be replaced, as regards transmission from eithersubstation, by an impedanceelement of resistance equal to the impedanceof the line. Any reactance effect, which is in practice small, may beeliminated by neutralizing reactance and, therefore, need not beconsidered. The condition, then, that the substation have maximum energyabsorption from the line is that its impedance as seen from the line bea pure resistance of value equal to the impedance of the line. Thiscondition is evidently equivalent to the following requirement; let anelectromotive naeeaas force be impressed on the substation termiappliedto the line terminals.

the impedance of the substation as seen I it follows that pedance ofthe'line; then the energy consumed in the substation shall be equal tothe energy consumed in said resistance.

Further, line and auxiliary resistance are conjugate by requirement (2),as hereinbefore stated, or in other Words, the auxiliary resistance isconnected to points of equal potential with respect to an electromotiveforce Moreover,

from the line should be equal to that of the line. Let, then, anelectromotice force E, be impressed through a resistance R on asubstation whose transmitter and receiver resistances are R, and B,respectively, and let the resultant currents in line, transmitter andreceiver be 1,, I, and I respectively; then the impedance across thesubstation terminals must be R as seen from the line and the totalresistance in series with E is 2B,, and Since the current in the line is1,,

E 2R, The total energy consumed may then be expressed by the formula13B, +1 11, +I,=R, 1 E, gs

. 4 Since the energy consumed by the substation is equal to thatconsumed by the resistance R and is therefore one half of the totalenergy consumed, it follows that requirement 4 may be formulated by thefollowing equation:

.( 4) (a a 1 4R This equation states that the energy consumption in theresistance R is equal to that in the substation, and that the substationis equivalent, as seen from the line, to a resistance of value R4.

Similarly, if transmitter and receiver are conjugate the condition thatthe transmitter shall have its maximum output to line and auxiliaryresistance may be formulated as follows: Let an electromotive force E,in the transmitter produce currents 1,, I and I in transmitter, line andauxiliary resistance. Then, for maximum output, it follows that I =11 IR I "*R (o (a a 3 4R1 Equation (2) is the analogue of equation (1) andmay be interpreted as follows by reference to Fig. 1: Let. thetransmitter be disconnected from terminals 5 and 6 and let the impedancebe measured across said terminals. Then if equation (2) is satisfied theimpedance so measured is equal to the impedance of the transmitteritself. In

other words, the impedance of the combination, as seen from thetransmitter is equal to that of the transmitter itself.

As hereinafter shown forthe particular embodiments of my invention,equation (2) follows as a consequence of the conditions of doubleconjugacy and equation (1). Therefore the foregoing four requirementsimpose but three restrictions on the substation.

To complete the general discussion itremains to consider the energydivision between receiver and transmitter when receiving, and betweenline and auxiliary resistance when transmitting. Let W be the totalamount of telephonic energy developed by the transmitter at thetransmitting substation; then, by equation (2), 1/2W is the amount ofenergy delivered to line and auxiliary resistance. .Let the amount ofenergy taken by the auxiliary resistance be m times that taken. by theline, then the amount of energy taken by the line is 1 zms so that thetransmitting efliciency is measured by 1 (iii) Of the total energydelivered to the receiving substation, let the transmitter absorb ytimes that absorbed by receiver; then the receiving efliciency ismeasured by The over-all etliciency from transmitter of one station toreceiver of communicating station is clearly proportional to the productof the transmission efiiciency and receiving efiiciency; therefore theover-all. efficiency is by formulae (3) and (4):

if m and y were independent, clearly the over-all eliiciency would be amaximum for mzg zO. For all substations embodying the principles of myinvention it may be readily shown. howey' er. that m and y are connectedby the relation sag =1. Eliminating a: from the above formula by meansof this relation, the expression for the over-all efliciency becomes(1), equation (2) is satisfied. Let S denote the current produced inbranch or ele ment 1 by a unit electromotive force in branch 1, S thecurrent produced in branch 2 by a unit electromotive force in branch 1,etc. Then by the conjugacy of branches 1 and 2 and branches 3 and 4, itfollows that Also by equation (1) 1 (S44)ZR4 n z 40 1 4 and by equation(2) 1 I (S11) 2R1 (S18) 2R3 (8102B; ZR l 1 Now it is a fundamentalprinciple which is deduciblo from elementary algebra, that S S That is,the current set up in branch 1 by a unit clcctromotivc force in branch 4is equal to the current set up in branch 4 by a unit clectromotive inbranch 1. Multiplying equation (1) by R, and equation (2) by R, andsubtracting it follows that:

Now in accordance with the notation adopted in this specification, theenergy consumed in branch 3 is m times that consumed in branch 4 when anelectromotive force acts in branch 1 therefore Also the energy consumedin branch 1 is 3/ times that consumed in branch 2 when an electromotiveforce acts in branch 4; whence (S14) 1 y( 24) 2 Multiplying (b) and (0)(S18) 1 a Y/( z4) z a.

From (a) and (d) it follows at once that y Obviously the expressiongiven by formula (5) is a maximum when 11 1. This means that for a givenamount of telephonic energy developed in the transmitter at thetransmitting substation a maximum amount is usefully delivered to thereceiver at the receiving substation connected by the line, when y=l.Since the maximum amount of energy in the receiver is the primedesideratum of telephony, it would appear that the substation should bedesigned to make y=1. Another consideration, however, modifies thisconclusion somewhat, namely, the effect of line noise. Since the linenoise originates in the line the amount delivered to the receiver isproportional to 1 y (see equation 4) while the amount of energy aaaaredelivered from the transmitter of the communicating station isproportional to 1 y (see'equation 5). The ratio of the latter to theformer is and this increases as y increases beyond unity. It will beclear, then, that if y is made greater than unity the substationdiscriminates against line noise as compared against line noise isobtained with a small loss in over-all efficiency.

The above considerations as to over-all efficiency and discriminationagainst line noise may be formulated as 1( 1) 2( 2) for an electromotiveforce inserted in the line. In this equation y is to have" a value lyingbetween 1 and 1.5 preferably.

Proceeding now to a description of the specific-circuits, one form of myinvention is illustrated in Fig. 1 in which L represents a telephoneline terminating in a substation, comprising a transmitter T, a receiverR, an auxiliary resistance X and a two winding induction coil, thewindings of which are designated N and N The line L, transmitter T andreceiver R are connccted to a common terminal 5, the remaining terminals6 and 8 of the transn'iittor and line respectively being interconnectedby the winding N of the induction coil, while the terminal 7 of thereceiver and terminal 8 of the line are interconnected by the windingN,,, the coils N and N being oppositely wound. The auxiliary resistanceis bridged between terminals 6 and 7 of the transmitter and receiverrespectively. A condenser C may be provided if desired to prevent theflow of direct current in the receiver.

The operation during transmission is indicated in Fig. 2 in which thearrows represent the direction of current fiow at any given instant.When the transmitter T is operated, variations of current flow areproduced, the effect being equivalent to applying a variableelectromotive force to the tweets motive force tends to raise terminals6, 8 and 7 to the same potential, so that the current would tend todivide and part flow through the line and part throu h the receiver, theresistance X being in e ect short circuited. Owing to the highinductance of the coils, however, an electromotive force is induced inthe coil N of such value and direction as to reduce the terminal 7 tothe same potential as terminal 5 so that no current can flow through thereceiver, and a counterelectromotive force is induced in winding N, insuch direction as to prevent the full transmitter current I from flowingthrough the winding N As a result the current I, divides at terminal 6and a current I flows through the resistance X and winding N to terminal8, while the remainder (I I flows through the winding N to terminal 8,where the two currents combine so that a line current I, equal to thetransmitter current I flows over the line and back to the transmitter.

The action during reception is indicated in Fig.3 and is as follows:Upon the application of a receiving potential to the line terminals acurrent 1 flows from the line to terminal 8 where it divides and acurrent I, flows through winding N and receiver R to terminal 5, while acurrent I flows through winding Nb and transmitter T to the sameterminal where the two currents combine so that a current I, flows backover the line. No current flows through the resistance X which it willbe noted is connected across neutral points 6 and 7 of a Wheatstonebridge of which coils N and N", transmitterT and receiver R constitutethe legs. As the coils N and N, are of negligible resistance nodifference in potential exists between points 6 and 7 and the windings Nand N are so proportioned that the resultant induced, electromotiveforce shall be zero so as not to disturb this equality of potential. Theonly receiving energy wasted is that passing through the transmitter,this loss being comparable to the transmitter loss in the standard set.v

- The design formulae by which the various elements of the substationmay be proportioned to accomplish the above results and satisfy the fivefundamental requirements outlined in the firstpart of the specification,may be derived as follows:

The first requirement is that during transmission the receiver must beconjugate with respect to the transmitter, or in other words, thereceiver must beconnected across points of equal potential with respectto an electromotive force applied to the transmitter. Let the number ofturns of the windings of the induction coil connected between thetransmitter and the line be designated as a the turns between the lineand receiver be m,

and between the transmitter and receiver be a and let K be the potentialdrop er turn. Then from Fig. 2 it a potential be applied to theterminals 5 and 6 of the transmitter, the drop through N and the linemust be equal to V. So also the dro through X, N, and the line mustequal Therefore V=Kn +R,I, V:R 1 Kn, -]-R,,I The factor Kn being anincrease in potential instead of a fall in potential is given the minussign. From these equations we get Moreover since points 5 and 7 are atthe same potential the drop through X must equal the drop through N andthe line. Therefore 3 3 14+ 4 4 v From the last two equations it isclear that From this equation and equation (a) we get R l n +n 1, i 14 44 42 i The current flowing through winding N is 1 Moreover since currentI, is equal to current 1,, it is clear that the current flowing throughN is I,I

It, then, the impedances of the transformer windings are very high theresultant magnetizing current must be zero, whence 3 42 4 s) 14 ortransposing From this equation and equation (6) the following equationmay be derived:

42 14 The above equation satisfies the condition that the receiver mustbe conjugate with respect to the transmitter during transmission. Thenext requirement is that the aux iliary resistance must be conjugatewith respect to the line during reception so that no energy shall bewasted in the auxiliary resistance. If, then, in Fig. 3, a potential Vbe applied to the line terminals it is apparent that the potential dropthrough winding N and the receiver must equal the potential drop throughN and the transmitter. Therefore,

42+ 2 2 14+ 1 1 But since terminals 6 and 7 are at the same potential,

Moreover the resultant magnetizing current in the transformer must equalzero, whence From the last two equations we get as the expressionsatisfying the requirement of conjugacy between the auxiliary resistanceand line The next consideration is that of maximum absorption of energyby the receiver during reception. From the equation just developed it isapparent that during reception currents I}. and I are proportional to mand 42 respectively. It is also apparent from Fig. 3 that Thereforecurrent I, is proportional to n -f n Substituting these values inequation (1) we have as the expression for maximum absorption by thereceiver The requirement of over-all eflicieiicy and discriminationagainst line noise was expressed in equation (6). Substituting the samevalues in this equation we have The next consideration is that thetransmitter shall supply maximum energy to the line during transmission.From equation (0) above developed it is apparent that duringtransmission I is proportional to ra t-n and I is proportional to a FromFig. 2 it is also apparent that I :I so that 1 must be proportional to7?,42+1L14. Substituting these values in equation (2) we have as theexpression for maximum output of the transmitter 1 4: u) s 10 4 42 "14)I From equations 7 to 11 inclusive the following design formulae arereducible:

It will be observed that the values of the resistances of thetransmitter, receiver and auxiliary resistance are determined from theknown impedance R of the line and the energy division factor 3 If in thearrange- A modification is shown in Fig. 4 Which differs from the formshown ill Fig. 1 in the connections of thetransformer, the winding Ninterconnecting terminals 6 and 7 instead of 8 and 7 as in Fig. 1, thecoils being wound in opposite directions. The essential difference willbev clear from a comparison of \Figs. 1(a) and 4(a).

The action during transmission is indicated in Fig. 5 and-is as follows:.A variable electromotive force is set up in the transmitter T whichtends to raise terminals 6, 7 and 8 to the same potential owing to thenegligible resistance of windings N and N so the resistance X would be,in effect, shortcircuited, and the current flow would divide between theline and receiver. Owing to the inductance in the coils, however, anelectromotive force is induced in Winding N, of such value and directionas to lower terminal 7 to the same potential as terminal 5, so that nocurrent flows through the receiver. Consequently a current ofinstantaneous value I, flows through the transmitter to terminals 66',and a line current 1, equal to current I flows over the line and back tothe transmitter, while an induced current I flows in the local circuitincluding winding N terminals 66, resistance X and terminal 7.

The action during reception of signals is indicated in Fig. 6. Apotential applied to the line terminals causes a line current I to flowthrough coil N, to terminal 6 where it divides and a current I flowsthrough the transmitter while a current I flows through coil N, and thereceiver R back to the line. N 0 current flows through resistance Xwhich is connected across neutral points of a Wheatstone bridge, ofwhich the receiver and transmitter form two legs, the connection (36 ofzero resistance forming a third leg, and the coil N of negligibleresistance forming the fourth leg. No difference in potential normallyexists between terminals 6 and 7 and the windings N and N, are soproportioned that the resultant induced electromotive force is zero, sothat the equality of potential between 6 and 7 is not disturbed. Henceno energy is lost in the winding X, the only loss being that due tocurrent flowing in the transmitter T, which is commensurate with thetransmitter loss in the standard substation.

The only difference between Figs. 1 and 4 resides in the changedconnections of the transformer windings. As the first three equations ofdesign formulae A, give the several resistances R,, R, and R in terms ofLemma when n and m, represent the number of turns on coils N and Nrespectively. So also the number of turns n connecting the line andtransmitter is Substituting these values in the last equa tion offormulae (A) we have Let r be the ratio of the number of turns onwinding N. to the number on N',,. Then The connections of the inductioncoil windings may be still further varied as shown in Fig. 7. In thiscase the transmitter, receiver, auxiliary resistance and lineconnections remain as before, but the terminals 6 and 7 areinterconnected by a winding N, while terminals 7 and 8 are interconnected by Winding N both coils being Wound in the same direction.

The operation during transmission is indicated in Fig. 8; Upon theapplication of a variable electromotive force to the transmitter,terminals 6, 7 and 8 would be raised to the same potential, as windingsN and N, are of negligible resistance. Resistance X would then beshortcircuited and the current would divide between theline and reeeiver. Owing to the inductance of the coils, however, acounter-electromotive force is induced in coil N tending to oppose theflow of current in winding N due to the transmitter potential, andanelectromotive force is induced in winding N tendin to cause acurrentfiow from terminal 7 to terminal 8. The effect of these inducedforces is to reduce terminal 7 to the same potential as terminal 5 sothat no current flows through the receiver, while the current I flowingto terminal 6 divides and current I flows through resistance X, theremainder passin through winding N so that a current If, equal totransmitter current I flows through coil Nb and over the line.

During reception, as is indicated in Fi 9, a potential applied to theline termina s causes a current L to flow through coil N ,to terminal 7where it divides so that a current I flows through the transmitter and acurrent I flows through the receiver to common terminal 5, and from thispoint a current I flows back over the line. No current flows throughresistance X which is connected across neutral points 6 and 7 of a Wlleatstone bridge-similar to that described in connection with Fig. 6.No difference in potential would normally exist between points 6 and 7and coils N; and N, are so proportioned that the induced electromotiveforce in each shall be zero so that this condition shall not bedisturbed. No energy is therefore lost in the auxiliary resistance.

As in the modification of Fig. 4., the first three equations of designformulae (A) apply equally well to the modification of Fig. 7 and fromthese three equations the resistance of the transmitter, receiver andauxiliaryresistance may be determined in terms of the known lineresistance and the energy division factor y. The design formula for thetransformer may be derived from the fourth equation of design formulae(A) thus:

Letting Km we have A slightly different type of circuit is illustratedin Fig. 10. lhis circuit differs from that of Fig. 1 in that thetransmitter and line are interchanged, as will be readil apparent from acomparison of Figs. 1(a and 10(a). The coils and N of the transformerare oppositely wound, and ifdesired a condenser C may be provided in thereceiver circuit to prevent the flow of direct current. V

The operation durin transmission will be clear from Fig. 11. variableelectromotive force at the transmitter tends to raise points 6 and 7 tothe same potential, as the windings N and N, are of negli ibleresistance. Due to the induction of the coils, however, acountereleotromotive force is induced in winding N so that terminal 7 isreduced to the same potential as terminal 5, and no current flowsthrough the receiver, while the induced electromotive forces cause acurrent I, to flow through the resistance X from terminals 6 to 7 andthen through winding N Transmitter currentI joins this current at 8 sothat a current I +I flows through coil N and a current 1,, equal. to thetransmitter current flows over the line and back to the transmitter.

The action during the reception of signals is indicated in Fig. 12. Apotential applied to the line terminals causes a line current to flowthrough Winding N to terminalnegligible resistance, terminals 6, 8 and'7 would be at the same potential and no current would flow through theauxiliar resistance. The windings are so proportloned that no induced.electromotive force results from the flow of current through the Windings and the points 6 and 7 remain at the same potentialnotwithstandingthe inductive relation of the coils.

The design formulae may be obtained in a manner analogous to thatdescribed in connection with Fig. l. The first requirement is' that ofconjugacyof transmitter and receiver during transmission. From Fig. 11it is clear that since terminals 5 and 7 are at the same potentialwithrespect to terminal 6, the drop through the line is equal to the.

drop through the auxiliary resistance X. Hence R3I8=R4I4 'Since thetotal magnetizing current in windings N and N, is zero, and the currentflowing through winding N, is I +I,, we have 14 4 (12*14) a From theabove equations we get as the expressions satisfying the requirement ofI vcomugacy of transmitter and receiver,

The equations from which the expression for conjugacy of line andauxiliary resist- From this it follows that K, the drop per I turn'iszero. If then a potential V be applied to the line terminals the dropthrough the transformer windings may be disregarded and we haveV=]Et,I,=R I

As the total magnetizing current in the transformer is zero As I ,=I,+Ithis expression may be writ ten 1 14 2( 12- 14)- From theseseveralequations We get, as

teasers the expression for conjugacy of line and auxiliary resistance 5N 14 2 "12 14 (16) It has just been shown that 4 1+ 2 and From theseexpressions it is obvious that during reception 1,, I and I, areproportionalto a -n a and m, respectively, Substituting these values inequation (1) we have as the expression for maximum absorption of energyby the receiver,

Substituting the same values in equation (6) the expression for over-allefficiency and discrimination against line noise becomes From theseequations it is clear transmission currents 1,, Land portional ton,,-n,,, 7L and-'n -wt respectively. Substituting in equation (2) theexpression for maximum output of the transmitter becomes From equations(15) to (19) inclusive, the following design formulae may be obtained:

that during R2: a y 4 Lettin the ratio of the number of turns on coil tothe number on coil N, be designated as r, the last equation may bewritten In Fig. 13 is illustrated a variation of the modification ofFig. 10, in which the connections of the induction coil windings arevaried in a manner analogous to that shown in Fig. 4, as Will be clearfrom a comparison of Figs.1(a). 4(a), 10(a) and 13(a). The coils N and Nare oppositely wound.

The operation durin transmission is illustrated in Fig. 14:- ariationsof potential at the transmitter tend to raise terminals 8, 6 and 7 tothe same potential so that 1,, are prothrough winding the current tendsto. divide between the line and the receiver, the auxiliary resistancebeing in effect shortcircuited. Due to the inductive relation ofwindings N and N however, an electromotive force is induced in windingN, of such direction and value as to reduce the potential of terminal 7to that of terminal 5, at the same time balancing the electromotiveforce at terminal (3 tendin to shunt part of the current away from t eline and through winding N, to the receiver. As a result a current 1,equal to current I, flows to the line, while the induced electroinotiveforce in the winding N causes a current I, to flow from terminal 6 to 7through the auxiliary resistance X. As terminals 5 and 7 are at nocurrent flows through the receiver.

During reception, as indicated in Fig. 15, a potential applied to theline terminals causes a current I to flow to terminal 6 where-itdivides, part of the current flowing N and the transmitter, and partflowing through winding N an the receiver. As coil N is of negligibleresistance points (Sand 7 are at the same potential. Thecoils are soproportioned that no induced electromotive force results, andterniinals6 and 7 remain at the same potential with the result that no currentflows through the auxiliary resistance while currents I. and I traversethe transmitter and receiver respectively.

As the various elements in this modifical'lOll are connected lll the52111101 manner 21S lll Fig. 10, with the exception of the windings ofthe transformer, it is obvious that the first three equations offormulae (B) apply also to this modification. The formula for the designof the transformer ma be derived from the fourth equation of ormulae (B)as follows:

Leta, and a represent the number of turns on windings N and N Thenfinglei'fi 71/ 11.

Letting the ratio of the number of turns on winding N to that on N,equal 1* we have .from the fourth equation of formulae (B),

,16. Both coils N 'and N are in this case wound in the same direction.

The operation during transmission will be clear'from Fig. 17. A.potential variation at the same potential dicatecl in the transmittertends to raise terminals 8, 7 and 6 to the same potential, so thatresistance X would be in effect shunted and the current flow would tendto divide between the receiver and the line. Due to the highselfimpedance of winding N a counter-electromotive force is induced whichreduces the potential of terminal 7 to the same potential as terminal 5so that no current flows through the receiver, while an electrmnotiveforce is induced in cell N in a direction tending to cause a How ofcurrent. through the coil from 7 to (5, so that a current I, howsthrough the resistance X, and a current I, flows over the line and backto the trans nutter.

The operation during reception is indicated in Fig. 18. line terminalscauses a current I, to fiowto terminal 6 and through winding N ofnegligible resistance to terminal '7 which would be at substantially thesame potential as terminal 6, the resistance X being in effectshortcircuited. At terminal 7 the current divides, and a current I flowsthrough the winding N and the transmitter, While a current I flowsthrough the receiver R. The coils are so proportioned that there is noinduced electromotive force and consequently terminals (3 and 7 remainat the same potential so that no current is wasted through the auxiliaryresistance.

Design formulae (B) also apply to this n'mdiiication. The fourthequation of design formulae (B) may be expressed in terms of the ratioof the number of turns on winding N to the number err-winding N, asfollows:.-:

Let

Then the above expression becomes ya-1 (22) A third general type whichdiffers from the general type illustrated in Fig. 10, in that thetransmitter and receiver are interchanged, is illustrated in Fig. 19.The difference between the modifications of Figs. 1, l0 and 19 will bereadily apparent from a comparison of the simplified diagrams of Figs.1(a), 10(a) and 19(a). The coils of the transformer are wound inopposite directions.

The operation during transmission is Fig. 20.

Assuming a variable A potential applied to the CLO electromotive forceapplied at the transmitter, since the resistance of windings N and N, isnegligible, terminals 6 and 8 would be at the same potential as terminal7 thus causing current flow through the line and receiver in parallel,were it not for the fact that windings N and N have high inductance.Owing to the inductance of the windings,however, a counter-electromotiveforce is set up in winding N5, and an electromotive force tending tocause current flow from 8 to 6 is set up in winding 1 These forces coactto reduce terminal 8 to the same potential as terminal 5 through thereceiver, and at the same time the potential of terminal 6 is loweredbelow that of terminal 7 so that a current I flows through theresistance X from 7 to 6. Due to the difference in potential betweenpoints 6 and 5 a current I flows over the line. No current flows throughthe windings N and N and the three currents 1,, I and I are equal, sothat a current flows serially through the transmitter, auxiliaryresistance and the During reception the operation as indicated in Fig.21 is as follows :-A potential applied to the line causes a current I toflow to terminal 6, and through winding N, to terminal 8. As thewindings N and N b are of negligible resistance points 8 and 7 have thesame potential as terminal 6, so that no current flows through theauxiliary resistance X, while a current I flows through the receiver,and current I flows through winding N and the transmitter. The coils areso proportioned that no resultant electromotive force is induced in thewindings and consequently terminals 6 and 7 remain at the same potentialand no energy is wasted in the auxiliary resistance.

The design formulae for the modification of Fig. 19 may be derived in amanner similar to that discussed in connection with Fig. 1. Thus fromFig. 20 it is apparent that and whence I so that no current flows Alsoit is clear from Fig. 21 that V: K77'42+R2I2 and V=K7Z42+IQZ12+R1I1 As Kequals zero it is apparent that R1I1:R2I2

From the theory of transformer action, it is clear from Fig. 21 that 4'42 1 12 which may be expressed Substituting the same values in equation(6) we have as the expression for energy division and over-allefficiency It has been shown that during transmission I :I =I Hence inequation (2) these symbols may be canceled and equation (2) reduces toas the expression for maximum output of the transmitter.

Simplifying and collecting equations (23) to (27) inclusive, thefollowing design formulae are obtained:

4: '1! Denoting the ratio of the number of turns on winding N to thenumber on N as 1' we have *=r 11,0 774, Therefore the fourth equationmay be expressed 22 illustrates a modification of the general type ofsubstation illustrated in Fig. 19, in which the transformer connectionsare interchanged in a manner analogous to that of Figs. 4 and 13, aswill be clear from a comparison of the corresponding schematic diagrams.The common termina of the two coils is connected to terminal 6 and theirother terminals are connected to terminals 7 and 8, the coils beingoppositely wound.

The operation during transmission is indicated in Fig. 23. A variableelectromotive force applied to the transmitter tends to raise points 7,6 and 8' to the same potential so that current tends to flowthroughwinding N, to terminal 6 and divide between theline and the transmitter.Owing to the high self inductance of coil N a counter-electromotiveforce is induced in said winding which prevents any flow of current from7 to 6, through said Winding, and reduces the potential of terminal 6below that of terminal 7, while an electromotive force is induced inwinding N which reduces terminal 8 to the same potential as terminal 5.As a result no current flows through the coil N and the receiver orthrough the coil N while a current I flows through the resistanceX'between terminals 7 and 6, and a current I flows over the line.Currents 1,, I and I, are equal and hence a current flows seriallythrough the trans- I mitter, auxiliary resistance and line.

' mulae (C) apply equally well toFi .22.

During reception, as shown in Fig. 24, a potential applied to the lineterminals causes a current I to flow to terminal 6, where it divides anda current I flows through the winding N, and the transmitter, while acurrent I flows through the Winding N and the receiver. As the windingsN and N, are of negligible resistance terminals 6, 7 and 8 are atsubstantially the same otential. The. coils are so proportioned t at noresultant clectromotive forces are induced therein and the points 6 and7 therefore re main at the same potential, so that no current flowsthrough the auxiliary resistance.

As the elements in this modification, aside from the transformerconnections, are arranged the same as in-Fig. 19 it is evident that thefirst three equations of design e fourth equation for Fig. 22 may ederived from the fourth equation of design formulae (C) as followsLetting the ratio of the number of turns on winding N to the number on Nbe Hence A third modification of the general form illustrated in Fig. 19is shown in Fig. 25. In this case. the connections of the induction coilare interchanged in a manner analogous to that shown in Figs. 7 and 16,the common terminal being connected to 7 and the other terminals too and8. The coils in this case are wound in the same direction, the otherelements remaining as before.

The operation during transmission as shown in Fig. 26. is as follows Avariable potential applied to the transmitter tends to bring terminals7, 6 and 8- to the same potential, thereby shunting re.- sistance X andtending to cause the current flow to divide between the line and thereceiver. Owing to the high inductance of windings N and N however, acounterelectromotive force is induced in Winding N and lowers terminal 8to the same potential as 5 so that no current flows through thereceiver. Likewise a counter-electromotive force is induced in winding Nwhich prevents the flow of current therethrough and lowers the potentialof terminal 6 so that a current I, flows through the auxiliaryresistance, and a current I, over the line. As 1,, and I, are equal acurrent flows serially through the transmitter, auxiliary resistance andline. and no current traverses the receiver or the induction coilwindings.

During reception, as indicated in Fig. 27, a potential applied to theline terminals causes a current I, to flow through winding N to theterminal 7 where it divides and current I flows through the transmitter,and

current I flows through the winding N 11

